HYADES combines the in-orbit demonstration of a new technology with its first ambitious scientific application. The satellite will investigate water and its isotopic composition in comets, active asteroids and interstellar objects by observing far-ultraviolet radiation that cannot be detected from Earth’s surface.
In the proposed configuration, light will be collected by a 20 cm Ritchey–Chrétien telescope feeding two ultraviolet cameras: HDi for observations of hydrogen and deuterium, and OHi for observations of the hydroxyl radical, OH. The mission is planned for a low, Sun-synchronous Earth orbit at an altitude of approximately 500–600 km.
Why far ultraviolet?
Sublimating ice releases water molecules into the gaseous coma surrounding a comet’s nucleus. Solar radiation breaks these molecules apart, producing, through successive stages of photodissociation, atoms of hydrogen and its heavier isotope, deuterium.
These atoms absorb and re-emit sunlight in the Lyman-α line near 121.6 nm. This exceptionally strong emission line allows even small amounts of gas to leave a detectable signature. Comparing hydrogen and deuterium emission, using models of water photodissociation, atomic motion and interactions with radiation, will allow scientists to determine the D/H ratio.
However, Lyman-α radiation lies in the far ultraviolet and is absorbed by Earth’s atmosphere. These measurements must therefore be made from space—the environment for which HYADES is specifically designed.
Extracting a faint signal from a bright background
Placing a telescope above the atmosphere does not remove every observational challenge. Hydrogen surrounding Earth forms an extended geocorona that also emits Lyman-α radiation. Additional emission comes from hydrogen in interplanetary space. This bright background can overwhelm the much fainter signal from the target.
Deuterium measurements are particularly demanding. Its Lyman-α line is separated from that of ordinary hydrogen by only about 0.033 nm, and its signal is much weaker. The instrument must therefore distinguish the emission of the two isotopes while limiting unwanted radiation.
A key component of HDi will be the gas absorption filter being developed within the project. It exploits the ability of gas to absorb light in extremely narrow spectral lines. This allows selected components of the incoming radiation to be suppressed while preserving the ability to image an extended gaseous coma.
The filter has two main functions: reducing the background that limits observations and enabling hydrogen and deuterium emission to be distinguished. During deuterium measurements, suppressing the much brighter hydrogen emission from the comet itself will be particularly important. Measurement performance also depends on observing geometry: the motion of gas relative to the observer shifts its spectral lines through the Doppler effect. Selecting suitable observing windows will therefore be an integral part of instrument operations.
Scientific instruments
The primary scientific instrument will be HDi—the Hydrogen and Deuterium Imager, which will image hydrogen and deuterium in the Lyman-α line. It will measure both the amount of gas released and its distribution around the target. Observations obtained with different filter settings will allow the weak deuterium contribution to be isolated.
The complementary OHi—OH Imager will observe hydroxyl emission near 309 nm. In cometary comae, OH originates almost exclusively from the breakdown of water, making it a particularly reliable tracer of its presence.
This matters because atomic hydrogen can also originate from other compounds. Near-simultaneous OH observations will test whether the hydrogen measured by HDi is consistent with the amount expected from water photodissociation. Any excess will indicate a contribution from other sources. The two cameras will therefore provide complementary information needed for a reliable interpretation of D/H measurements.
Faint ultraviolet light will be recorded by detectors based on microchannel plates. These amplify the signal generated by an individual photon, allowing it to be registered. Low noise and the ability to accumulate light over long observations are essential for studying faint, extended gaseous envelopes.
Instrument development includes laboratory testing with synchrotron radiation. At the SOLARIS National Synchrotron Radiation Centre in Kraków, access to a beam covering the Lyman-α spectral region has been established at the URANOS beamline. This enables precise measurements of the transmission properties of filter prototypes.
The first measurement campaign took place in February 2025. Further experiments are developing and validating the technology ahead of its application in space.